Fire Interval Effects on Successional Trajectory in Boreal Forests of Northwest Canada |
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Authors: | J F Johnstone F S Chapin III |
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Institution: | (1) Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA;(2) Present address: Department of Geography and Environmental Studies, Carleton University, Ottawa, ON, K1S 5B6, Canada |
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Abstract: | Although succession may follow multiple pathways in a given environment, the causes of such variation are often elusive. This
paper describes how changes in fire interval mediate successional trajectory in conifer-dominated boreal forests of northwestern
Canada. Tree densities were measured 5 and 19 years after fire in permanent plots and related to pre-fire vegetation, site
and fire characteristics. In stands that were greater than 75 years of age when they burned, recruitment density of conifers
was significantly correlated with pre-fire species basal area, supporting the expectation of stand self-replacement as the
most common successional pathway in these forests. In contrast, stands that were under 25 years of age at the time of burning
had significantly reduced conifer recruitment, but showed no change in recruitment of trembling aspen (Populus tremuloides). As a result, young-burned stands had a much higher probability of regenerating to deciduous dominance than mature-burned
stands, despite the dominance of both groups by spruce (Picea mariana and Picea glauca) and pine (Pinus contorta) before the fire. Once initiated, deciduous-dominated stands may be maintained across subsequent fire cycles through mechanisms
such as low on-site availability of conifer seed, competition with the aspen canopy, and rapid asexual regeneration of aspen
after fire. We suggest that climate-related increases in fire frequency could trigger more frequent shifts from conifer to
deciduous-dominated successional trajectories in the future, with consequent effects on multiple ecosystem processes. |
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Keywords: | alternative community states disturbance effects fire return interval Pinus contorta Picea mariana Populus tremuloides postfire regeneration successional trajectory |
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